A. Field of the Invention
This invention relates to ring spinning frames for processing staple fibers in the form of a continuous strand of loosely assembled crimped staple fibers with little twist (roving) or no twist (silver) to provide twisted yarn. For convenience, the term "sliver", as used herein, includes roving which is an intermediate state between sliver and yarn. More particularly, the invention relates to an improved apparatus for the drafting section of ring spinning frames whereby improved spinning performance in terms of breaks ("ends down") per 1000 spindle hours and more uniform yarns can be achieved without sacrificing productivity.
B. Description of the Prior Art
Ring spinning frames and their operation are well-known in the art. Conventional ring spinning frames comprise a plurality of individual spinning positions or stations, each having a drafting section for drafting a sliver to a desired size (e.g. cotton count), an upright rotatable spindle for inserting twist in the sliver and a ring-and-traveler take-up mechanism for winding the twisted sliver (yarn) onto a bobbin mounted on the spindle.
The drafting section comprises a pair of rear nip rolls for metering the input of the sliver and a pair of front nip rolls for drafting the sliver. One roll of each pair is a cot roll and the other roll is a serrated metal roll. The distance between the nip of the rear rolls and the nip of the front rolls is slightly greater than the length of the staple fibers in their extended configuration so that the individual fibers of the sliver are not stretched during drafting. The front rolls are operated at a peripheral speed considerably greater than that of the rear rolls. The drafting section also includes a drafting control means typically comprising a pair of rotatable aprons (e.g. a Casablanca apron system) positioned between the rear rolls and front rolls. The aprons are operated at a peripheral speed slightly greater than that of the rear rolls so as to control and straighten out the fibers of the sliver. However, during operation of the drafting section, a region of the sliver approximately 1.25 inches (3.18 cm) in length extending from the exit end of the aprons to the nip of the front rolls is not controlled. Instead of using aprons as the drafting control means, one or more pair of nip rolls may be used. A free-floating, funnel-shaped piece (condenser) through which the sliver passes during operation of the frame is positioned between the drafting control means and the front rolls to condense the sliver fibers into a narrow ribbon as the sliver approaches the front rolls.
In commercial practice of ring spinning frames, the spindle is operated at its highest practical mechanical speed (rpm) and the front rolls are operated at a peripheral speed that will insert the desired twist into the sliver. The rear rolls are then operated at a peripheral speed such that the ratio of the peripheral speed of the front rolls to that of the rear rolls (i.e. draft ratio) does not exceed 28:1. Under the foregoing conditions satisfactory processing of the sliver in terms of ends down and product uniformity in terms of thick and thin regions along the length of the resulting product yarn. However, as the draft ratio is increased from a value of 28:1, the yarn becomes increasingly less uniform (i.e. has more thick and thin regions) and sliver breaks become more frequent until the process can no longer be run.